Author sure has a funny way of drawing a 45 degree angle !
And conspicuously does not draw a hint of a hinge at B.

Furthermore, the horizontal line at 3 m is supposed to designate something sturdy that has water above and air below ? Quite something different from the liquid surface at 4.2 m; more like the stuff the gate itself should be made of. Deceptive drawing, incompetence, sloppiness ?